Process of flotation concentration of ores



Patented Apr. 11, 1939 PATENT OFFICE PROCESS OF FLOTATION CONCENTRATION OF ORES John Edward Hunt, Jackson, Calif.

No Drawing. Application March 12, 1937,

Serial No. 130,529

7 Claims.

This invention relates to the flotation concentration of ores in general. In particular it relates to the flotation of ores which contain carbonaceous or graphitic substances associated with its gangue components, a condition that is common for ores which occur in or were formed in sedimentary or metamorphic rocks.

It is well known that the gangue slime in many ores has a tendency to float and remain mixed 10 with the valuable ore minerals and also a tendency to coat or envelop the valuable ore minerals and thus render their recovery by flotation less complete.

By this invention the harmful effects of the 16 gangue slime in an ore are overcome or avoided during flotation and at the same time the surfaces of the sulfide minerals of metallic luster, tellurides and metallics are prepared for recovery by the flotation reagents commonly employed for this go purpose.

The gangue minerals in ores are often coated with a natural carbonaceous or graphitlc substance which resulted from the decomposition of organic (vegetable and animal) matter that was 35 included during the rock forming period. The

most pronounced carbonaceous coatings are usually with ores found in shales, schists and slates but such coatings occur to a greater or lesser degree in all sedimentary or metamorphic rocks. 30 -This carbonaceous decomposition product is a natural flotation agent and minerals or rock fragments which are covered with it, usually the gangue portion of an ore, will float with an ease comparable to that of sulfide minerals. Among .35 the deleterious effects, due to the flotation of such valueless carbonaceous gangue minerals or rock fragments, is the reduction of the sulfide and metallic (gold and silver) grade of the flotation concentrate by increasing its insoluble assay and M thereby adding to the cost for its subsequent treatment. Further, the flotation of carboncovered gangue or graphite prevents the successful application of the cyanide process to the flotation concentrate from ores in which gold and 45 silver are the only metals of economic importance. The harmful effects, in cyanidation, of the carbon coated minerals that are floated with the concentrate, come from their ability to cause premature precipitation of gold and silver from a cyanide 50 solution, and from their tendency to film the gold and silver-bearing minerals thereby preventing effective action by the cyanide solution.

Heretofore, no method has been known or used for controlling and preventing the flotation of these carbon-coated gangue particles, but my dis- .covery now provides a simple and effective process for accomplishing this desirable end. By the present invention I employ for the flotation concentration of ores containing deleterious gangue or containing minerals or rock fragments that are 5 coated with any natural carbonaceous substance, sulfur dioxide gas, or any other natural or manufactured reducing gas, as, for example, hydrogen sulfide, carbon monoxide, natural gas and others, with or without mixing it with air, to inhibit such 10 deleterious gangue and carbon-coated mineralsand prevent their flotation during the period in which the sulfide and sulfo minerals and metallics are being floated from an ore pulp by the use of' any of the well-known methods of flotation. The specific reaction of a reducing gas on carbon coated gangue minerals is not definitely known but it appears to be largely a physical one consisting of adsorption or absorption of the gas around or into these minerals with the result that their 20 flotation tendency is overcome and their behavior, while the valuable constituents are being floated,

is then caused to .be similar to that of normal gangue minerals.

The effect of sulfur dioxide and other reducing gases on the non-carbonaceous gangue slime seems to be one of dispersion. When the gangue slime is in a state of dispersion, its ability to float and its tendency to cover or envelop the sulfide and other valuable minerals in the ore, are greatly reduced .or substantially eliminated. Reducing gases have a similar effect on other non-sulfide minerals of metallic luster such as pyrolusite, hematite, etc.

In addition to these inhibiting effects of a physical nature on carbon covered particles, graphite and other slime constituents of an ore, reducing gases, sulfur dioxide gas particularly, appear to have a further action, chemical in nature, on sulfide and sulfo minerals of metallic luster and metallic elements. This chemical action appears to consist of reducing any salts or chemical coatings on the surfaces of such minerals or metallics to a soluble state thereby leaving the surfaces clean and avid to contact the flotation agents usually employed for their recovery.

In the application of my process to carbonaceous or slimy ores I favor the use of sulfur dioxide gas, in preference to other natural or manufactured reducing gases, on account of its availability and cheapness and its marked effect on both the carbonaceous minerals, the slime gangue and the sulfide, sulfo and metallic constituents of an ore. The amount of sulfur dioxide gas required depends on the nature of the gangue and the amount of graphitic and carbon coated minerals or rock fragments that are mined with or included in the ore. Hence the requirement of sulfur dioxide gas is best determined by laboratory tests on each individual ore to be treated. The gas requirement, in terms of elemental sulfur equivalent, for effective inhibition of graphitic and carbon coated minerals or rock fragments, will vary from less than 100 grams per ton of ore up to several pounds per ton of ore. The pH or hydrogen ion concentration of the flotation pulp water will be lowered in proportion to the amount of sulfur dioxide gas needed to inhibit the carbonaceous minerals. The pH of the pulp water is usually on the acid side (less than pH 7.0) after the addition of the required amount of gas. In some cases, however, depending upon the natural alkalinity of both the ore and milling water, the graphitic or carbonaceous minerals and the gangue slime are effectively inhibited by the gas while the pulp water remains alkaline, or plus pH 7.0, and flotation of the valuable constituents of the ore is completed under these conditions. My process can be carried out when the ore pulp is either acid or alkaline but, when the reducing gas used is sulfur dioxide, the pH of the pulp water is usually below 7.0 after the required amount of gas has been added.

The equipment required for the application of my process, beyond that which is needed and ordinarily used in flotation practice, consists of a sulfur burner, or pyrite roaster, a blower and pipe ducts to convey the sulfur dioxide gas to the points of application. I prefer to obtain the sulfur dioxide gas by burning elemental sulfur in air and then using a blower to convey the sulfur dioxide-air mixture, under suflicient pressure to meet local conditions, to they points of application. The percentages of sulfur dioxide gas and air in the mixture, when burning sulfur, are thus under the control of the operator. The sulfur dioxide gas to be used in the application of my process may also be obtained by roasting a pyrite concentrate if such a source is economical under local conditions. When the flotation concentrate is to be cyanided, prior roasting often increases extractions and the sulfur dioxide requirement for flotation is thus made available by this step. The reducing gas may also be generated in the ore pulp itself by the action of two or more suitable chemicals. For example, if sulfuric acid and an alkaline or alkaline earth sulfite, bi-sulfite or thio-sulfate are added to an ore pulp, sulfur dioxide gas will be one of the products resulting from the interaction of these chemicals.

A favorable point of application for the sulfur dioxide-air mixture is at a pulp conditioning tank ahead of the flotation circuit. The size of this conditioning tank can be designed to provide the time of contact, of the ore pulp with the gas mixture, that has been determined by prior laboratory experiments. Additional gas may be added at a concentrate conditioning tank ahead of each cleaner circuit. A spray tower, in which the ore pulp drops from the top and is broken up by baffles during its descent, while the gas rises countercurrent to the spraying pulp, is another very effective method for gassing.

The usual reagents required for flotation may 'be added while the pulp is being conditioned with the gas mixture or just afterward, ahead of the flotation cells, and the sulfide or sulfo minerals and metallics are then floated in the usual manner while the carbon covered minerals and gangue slime remain inhibited and are dropped and carried out with the tailing. The speed of flotation for the sulfides and metallics in an ore is in creased after the gas conditioning, thus further insuring their separation from the inhibited carbon covered or graphitic particles and the dispersed gangue slime.

The sulfur dioxide-air mixture may be introduced directly into the flotation cells in cases where a short contact time is found sufficient to inhibit the flotation of the carbonaceous or nonsulflde minerals of metallic luster in the ore being treated. In such an event, the flotation of the sulfide and sulfo-minerals and metallics, using the common well known reagents, would be accomplished during the time the non-sulfide minerals were being inhibited by the sulfur dioxide-air mixture.

My process may be used for the re-treatment of concentrate produced from ores by any of the common flotation methods, rather than applying it directly to the crude oil. Its application to flotation concentrate that contains any amount of carbon coated minerals or slime gangue or nonsulflde minerals having a metallic luster does not differ essentially from the method in which it is applied to crude ore. However, many concentrates that are produced by common flotation methods contain a high percentage of carbonaceous or slime gangue or other non-sulfide minerals having a metallic luster and it may be necessary to re-gas it one or more times in order to remove these minerals that had previously escaped inhibition.

The following are given as specific examples to illustrate the application and advantages of my process for the flotation of ores:

Example I.A sample was taken at an operating plant and was the same as the feed to flotation, a large portion of the free gold having been previously recovered by amalgamation. The sample was divided into two equal portions. One portion was treated in accordance with the current flotation practice of the district and the other portion was agitated with a small stream of sulfur dioxide gas for a period of 10 minutes after which flotation was carried out in the same manner and with the same reagents as used with the first portion. The rougher flotation concentrate was cleaned twice using a 5-minute period of sulfur dioxide conditioning prior to each cleaning and the cleaner tailings were combined. The consumption of sulfur dioxide was equivalent to a consumption of 1.6 lbs. of elemental sulfur per ton of ore. The consumption of all other-reagents was the same for both portions.

The concentrate obtained from the first portion was 3.82% of the original and was a black and slimy product that was difficult to fllter and could not be easily filtered or cyanided. It contained 2.330 oz. of gold per ton. 48.3% of this concentrate was insoluble. The tailings contained a considerable amount of valuables. The concentrate obtained by my process from the second portion was only 1.29% of the original and was a yellowish granular product that could be cyanided. It contained 7.505 oz. of gold per ton. Only 9.2% of it was insoluble.

Example II.Flotation concentrate from a mother lode mill was treated by passing bubbles of sulfur'dioxide gas through the concentrate pulp for minutes while stirring it. The pulp was then charged into a laboratory cell and flotation of the pyrite, arsenopyrite and elemental gold was accomplished without the use of additional flotation reagents-as those which had been added at the plant persisted and sufliced for the reflotation. The re-floated concentrate was regassed for 5 minutes and again floated, after which a third cycle of gassing and floating was carried out. The tailings from the three flotations, containing the carbonaceous gangue of the original concentrate, were combined together for weighing and assaying. The concentrate after 3 gassings was filtered and washed and an assay sample was cut from the cake. The remainder of the concentrate was ground to pass 400 mesh and agitated with cyanide solution for 48 hours.

The re-floated concentrate weighed 34.8% of the original concentrate and was readily amenable to cyanidation. 8.6% of this concentrate was insoluble and this concentrate contained 6.025 oz. of gold per ton or 96.11% of the total gold.

The above new re-floated concentrate was reground to pass 400 mesh and. then agitated with 0.15% KCN solution for 48 hours at a dilution of 4 of solution to l of concentrate. The washed concentrate residue assayed 0.204 oz. gold per ton, which shows an indicated extraction by cyanidation of 96.6% of the gold in the re-floated concentrate.

Roasting the new flotation concentrate before cyanidation would provide the sulfur diomde gas needed for the purposes of my'process and provides' the possibilities of greater extraction by cyanide and of avoiding the necessity of further grinding before cyanidation.

I claim:

1. In the process of concentrating ores containing carbonaceous substances associated with gangue minerals by flotation, the steps which comprise treating carbonaceous ore pulp with a reducing gas, and subjecting said pulp to froth 40 flotation concentration.

2. In the process of concentrating ores containing carbonaceous substances associated with gangue minerals by flotation, the steps which comprise treating carbonaceous ore pulp with sulfur dioxide, and subjecting said pulp to froth flotation concentration.

3. In the process of concentrating ores containing carbonaceous substances associated with gangue minerals by flotation, the steps which comprise treating ore pulp containing mineral particles coated with carbonaceous material with a reducing gas, and subjecting said pulp to froth flotation concentration.

4. In the process of concentrating ores containing carbonaceous substances associated with gangue minerals by flotation, the steps which comprise treating ore pulp containing particles coated with carbonaceous material with sulfur dioxide, and subjecting said pulp to froth flotation concentration.

5. In the process of concentrating ores containing carbonaceous substances associated with gangue minerals 'by flotation, the steps which comprise treating carbonaceous ore pulp with a mixture of air and a reducing gas, and subjecting said pulp to froth flotation concentration.

6. In the process of concentrating ores containing gangue having carbonaceous material thereon by flotation, the steps which comprise treating carbonaceous ore pulp with a reducing gas sufliciently to decrease materially the tendency oi the gangue to float, and subjecting said pulp to froth flotation concentration.

7. The process which comprises concentrating metallic ore containing gangue having carbonaceous material thereon by froth flotation, treating the metal concentrate therefrom with a reducing gas and subjecting it to another flotation concentration.

JOHN EDWARD HUNT. 

